1. Field of the Invention
This invention relates to a kit, combination cutting and spacer guide, and method for use in determining the location of a plane for resecting a surface of a single condyle of the distal femur in a knee joint, relative to a reference surface on the proximal tibia, to prepare the femur for implantation of the femoral component of a unicompartmental knee joint prosthesis.
2. Description of the Prior Art
Accurate resection of a patient's bones is crucially important for success in an operation to implant a joint prosthesis. Accurate resection depends on accurate location of the resection planes. The orientations of the resection planes relative to the anatomical axis must also be accurately controlled to ensure proper alignment of the articulating surfaces of the joint after surgery, throughout the full range of joint motion. Proper location of the resection planes is important in achieving proper soft tissue balancing. Moreover, accurate location of the resection planes is necessary to minimize the amount of bone that is removed, while being sure to remove all bone tissue that is defective.
For implantation of a knee prosthesis, the location of the resection plane for the tibia is determined first, and the location of the femoral resection plane is then determined with reference to the tibial resection plane, generally after resection of the tibia. Accurate location of the tibial resection plane in a knee joint is commonly established using an alignment guide which includes an alignment rod which can be fastened to the tibia distally, close to the ankle. The rod extends along the tibia, parallel to the axis of the tibia. The tibial resection plane can then be defined relative to the tibial axis using a cutting block which can be attached to the alignment guide or to the proximal tibia. The tibial resection may then be performed along the tibial resection plane.
The distal femoral resection is performed along the distal femoral resection plane. The location of the distal femoral resection plane relative to the tibial resection plane will depend on factors which include the dimensions of the implant components that are to be used. Appropriate alignment and spacing of the femoral and tibial components should be maintained throughout the range of motion of the knee joint. This requires accurate location of distal, anterior and posterior femoral resection planes on the femoral component. In addition to the dimensions of the implant component, alignment and spacing can be affected by features of the patient's anatomy. For example, the alignment of the femoral resection planes can be arranged to provide a correction for varus and valgus deformations. It can also be required to ensure that the implanted femoral component is aligned with the natural condyle, to facilitate optimum coverage of femoral tissue and that the extent of overhang of the component beyond the femoral tissue is minimized.
When preparing a knee for a unicompartmental knee prosthesis, the relative positions of the tibial and femoral resection planes are significant for an additional reason: to ensure that the implant does not cause damage to the healthy side of the knee. Considering, for example, a unicompartmental knee prosthesis that is implanted on the medial side of a patient's knee, if too little bone is removed from the distal surface of the medial condyle, the native lateral condyle may be forced toward the tibial plateau, causing increased stress on the lateral meniscus and lateral side of the tibial plateau. Considering a unicompartmental knee prosthesis that is implanted on the lateral side of the patient's knee, if too little bone is removed from the distal surface of the lateral condyle, the native medial condyle may be forced toward the medial tibial plateau, causing increased stress on the medial meniscus and medial side of the tibial plateau. Either of these conditions could exist when the knee is in flexion, extension, or in both positions. Either of these conditions, in either or both positions, can damage or accelerate the deterioration of the healthy side of the knee. Moreover, in any case, if too much bone is removed from either side, the prosthesis may be too loose in flexion, extension or in both positions. Accordingly, it is important that the surgeon be able to assess the potential positions of both the condyle to be resected and the condyle to remain native relative to the resected and native proximal tibia, and it is important that the surgeon be able to make this assessment when the knee is in flexion and extension.